This invention relates to a direct positive light-sensitive silver halide photographic material, particularly, to a direct positive light-sensitive silver halide photographic material having a broad fogging exposure latitude, and also to a method of use of the same, particularly for carrying out development at a higher temperature.
In general, it is well known that a direct positive photographic image can be formed by using a light-sensitive silver halide photographic material (hereinafter merely called light-sensitive material) without requiring any intermediate processing steps or negative images.
Conventionally known methods used for forming a positive image with use of direct positive light-sensitive materials can be grouped into two types.
One type is a method in which, using a silver halide emulsion previously fogged, fog nuclei (latent images) of exposed portions are destroyed with utilization of the solarization or Herschel effect to obtain a positive image after developing.
Another type is a method in which, using an internal latent image type silver halide emulsion not previously fogged, the surface development is carried out after fogging treatment is applied, or while fogging treatment is applied, after imagewise exposure, to obtain a positive image.
The above internal latent image type silver halide photographic emulsion refers to a silver halide photographic emulsion that has sensitivity specks chiefly in the insides of silver halide grains so that latent images are formed in the insides of the grains when exposed to light.
The method of the latter type can provide a relatively higher sensitivity as compared with the method of the former type, and is suited for the application that requires a higher sensitivity. This invention particularly relates to the latter type.
Various techniques have been hitherto known in this technical field. For example, they chiefly include those described in U.S. Pat. No. 2,592,250, No. 2,466,957, No. 2,497,875, No. 2,588,982, No. 3,761,266, No. 3,761,276 and No. 3,796,577, and British Pat. No. 1,151,363, etc.
With use of these known methods, it is possible to produce light-sensitive photographic materials having a relatively higher sensitivity as the direct positive types.
With regard to the details of the mechanism by which the direct positive image is formed, any clear explanation can not be said to have been sufficiently made, but the process in which the positive image is formed can be understood to a certain extent, for example, from "Desensitization Action by Internal Latent Image" as discussed in Mees and James, The Theory of the Photographic Process, Third Edition, page 161.
More specifically, it can be presumed that owing to the surface desensitization action caused by the so-called internal latent images produced in the insides of silver halide grains by the initial image exposure, fog nuclei are selectively formed only on the surfaces of unexposed silver halide grains, and subsequently a photographic image is formed on the unexposed portions by the usual development.
As a means for selectively forming the fog nuclei, there are known a method called the light fogging (fogging by light) in which the fogging is effected over the whole area of a light-sensitive layer by exposing to light, and a method called chemical fogging in which the fogging is effected by use of an agent such as a fogging agent.
Of such methods, the chemical fogging method is disadvantageous in that it involves a severe condition such that the effect of the fogging agent can be obtained for the first time at a high pH value of pH 12 or more, so that there tends to occur the deterioration of the fogging agent owing to air oxidation to extremely lower the fogging effect.
On the other hand, in the case of the light fogging method, it involves no such severe condition as mentioned above, and is practically convenient. However, there remains technical problems in using it for various purposes in a comprehensive photographic field. Namely, since the light fogging method is based on the formation of fog nuclei by virtue of the photolysis of silver halide, its correct exposure irradiance or exposure amount may vary depending on the kind or properties of silver halide.
For example, Japanese Patent Publication No. 12709/1970 discloses a method of uniformly exposing overall area with use of low-intensity light. It teaches that a good positive image having a high maximum density and a low minimum density can be obtained by subjecting the overall area to lower irradiance exposure.
It has been also revealed as a result of the studies made by the present inventors, that in order to obtain a relatively good positive image, it is necessary to apply fogging exposure of a relatively low irradiance within a certain range of intensity. Thus, no sufficient maximum image density can be obtained with an irradiance lower than this range even if a sufficient exposure is applied, and also, with an irradiance higher than this range, there may occur a phenomenon that the maximum density decreases and the minimum density increases in proportion to intensity, which is so-called intensity reciprocity failure in light fogging.
Accordingly, it is desirable to control intensity and time for the overall uniform exposure by suitably changing them depending on the internal latent image type light-sensitive materials, development processing conditions and types of light sources to be used, so as to obtain a best positive image.
It is also possible to form a positive image of low contrast with use of the internal latent image type direct positive light-sensitive material. In this instance, it is well known to mix silver halide grains of different sensitivity or to laminate silver halide emulsion layers of different sensitivity in order to control the gradation.
For example, Research Disclosure (hereinafter abbreviated as R.D.) No. 15162 and U.S. Pat. No. 4,035,185 disclose that, in addition to mixing of silver halide grains having different grain sizes, silver halide grains having approximately the same grain sizes but different sensitivities or different gradations are mixed to control the gradation.
However, from our extensive studies, it was revealed that since the silver halide grains having different sensitivity from each other are also different in not only the sensitivity at the time of image formation but also the sensitivity to the overall exposure at the time of light fogging, the optimum conditions (or the optimum exposure range) for each overall exposure may also become different, and therefore, light-sensitive photographic materials in which silver halide grains having different sensitivity from each other are combined, has characteristics that the optimum exposure range may become narrower depending on its combination since the overlapped exposure region of each silver halide grains only can be the optimum exposure range.
However, descriptions as to the characteristics described above can not be found at all in the above R.D. No. 15162 and U.S. Pat. No. 4,035,185.
As a result of further studies on the above, it became clear that the above property is remarkable when the development processing is carried out at a high temperature, particularly at 30.degree. C. or more, and thus it was revealed that practical problems may arise in such an occasion. More specifically, it was revealed that when a internal latent image type light-sensitive material containing two or more kinds of silver halide grains having different sensitivity are subjected to overall exposure to apply light fogging and development processing (particularly at 30.degree. C. or more) is carried out, if a local non-uniformity of the exposure or a variation with time in the light intensity of a light source is caused at the time of light fogging, there are disadvantages that the density of the resulting positive image tends to vary and the gradation may also change to make it difficult to obtain a good image.